A few of the more well-known comets have been mentioned; but a year rarely passes in which at least one comet is not discovered, though often only a small specimen, sometimes even tailless and hairless. No doubt many others pass unseen. The large and grand ones only come to view now and then. What are comets and meteorites made of? Respecting comets, a good many ideas are put forth, and a good many guesses are made, as to “burning gas,” “luminous vapor,” “beams of light,” and so on. But in truth, little is known about the matter. Respecting meteorites, we can speak more certainly. A good many meteorites have fallen to earth as aerolites, and have been carefully examined. They are found to contain nickel, cobalt, iron, phosphorus, and sometimes at least, a large supply of hydrogen gas. Not only do solid aerolites fall half-burnt to the ground, but even when the meteorites are quite consumed in the air, the fine dust remaining still sinks earthward. This fine dust has been found upon mountain-tops, and has been proved by close examination to be precisely the same as the material of the solid aerolites. A luminous body of sensible dimensions rapidly traverses space, diffusing on all sides a vivid light--like a globe of fire of which the apparent size is often comparable with that of the moon. This body usually leaves behind it a perceptible luminous train. On or immediately after its appearance it often produces an explosion, and sometimes even several successive explosions, which are heard at great distances. These explosions are also often accompanied by the division of the globe of fire into luminous fragments, more or less numerous, which seem to be projected in different directions. This phenomenon constitutes what is called a _meteor_, properly speaking, or a _bolide_. It is produced during the day as well as the night. The light which it causes in the former case is greatly enfeebled by the presence of the solar light, and it is only when it is developed with sufficient intensity that it can be perceived. In the British Museum there is a superb collection of meteorites. They have been brought together from all parts of the earth, and vary in size from bodies not much larger than a pin’s-head up to vast masses weighing many hundred pounds. There are also many models of celebrated meteorites, of which the originals are dispersed through various other museums.